OpenBTSRFDevelopmentPlans

OpenBTS RF Development Plans

These are our ideas for future RF development. If you are working on the RF
task, feel free to edit and comment in this document.

Bear in mind that this device may not need FCC certification in its intended
deployments.

Transmit Chain

USRP -> tx filter -> PA -> duplexer -> cables and
antenna

The tx fitler provides suppression for out-of-band noise and is matched a
standard GSM downlink band. The current candidate design is a stripline
interdigital filter or low-cost cavity resonator (see below).

The duplexer proposal is a 2-cavity 1/4-wave design built from paint cans,
coffee cans or large-diameter copper pipe. We're not joking here. We don't want
a $4k hunk of precision machine work in the middle of a "low-cost"
system.

Receive Chain

antenna and cables -> duplexer -> rx filter -> preamps ->
USRP

The preamps need to provide at least 20 dB of gain to overcome noise inside
the USRP. They need to be cheap, too. And they need enough headroom to overcome
head-far problems. Even though GSM support active uplink power control, we do
not want to be entirely reliant on it. The good news is that with the antenna up
on a tower, there's a limit to how close handsets can be.

The rx filter is similar to the tx filter above, but passes the uplink band.
In this side of the radio, though, insertion loss and a efficiency are more
important.

USRP Changes

We are planning to build a new
radio board based on the USRP, but specifically tailored for GSM
applications.Right now, OpenBTS
uses a USRP with two RFX daughter cards. For actual deployment, it would be
better to have a single board that combines all of these features:

a USRP-compatible control section, modeled on the USRP motherboard

two RFX-style receive sections with better RF isolation

one RFX-stype receive section

an OCXO running at a multiple of 13 MHz

on-board GSM-specific filters, either SAW or ceramicThis board would
still be software-compatible with the USRP, allowing developers to continue to
use USRP-based kits for desktop testing.

Other Details

Antenna

For omni cells, we can probably get a lot of service from marine cellular
band antennas, built to be installed on boats and yachts. They are tough,
efficient and only about $100 each. For sectorized installations, there are a
number of panel style antennas on the market that should be suitable, including
inexpensive designs built for the US 900 MHz ISM band.

Connectors

We only want two connector types in our RF system: SMA for low power and N
for high power.